These devices allow optimal measurements when the distance between the point source and the aperture is also optimal. The order of magnitude of the distances is in the range from 0.1 to 1 mm, so that variations in the distance considerably impair the pre-established optimization. Such variations in distance may occur intentionally or accidentally by the user.
Another disadvantage of the prior devices is that after application of the direct current voltage to the Geiger-Muller-counter, the properties of the device continue to vary for a certain period of time. A "built-up" state is obtained only after a longer period of time. In the prevailing technology this is disadvantageous because the beta rays yield only statistically detectable measuring results, so that the statistics are superposed by another time dependent process.
In addition, the disadvantage of the prior devices is that their changing out is very time-consuming. There are various measuring problems (different materials on different films, different thicknesses, different apertures for differently curved measuring objects, and so on). Sometimes promethium is better suited as radiator, sometimes tallium, sometimes strontium.
Finally, the prior art devices do not take into account that every radiator represents an individual with respect to the radiating isotope and that every device must be tuned to this individual.